First of all, it sounds like you need to get a handle on your IT purchasing practices. If you're doing things right, 4 years shouldn't result in more than 2-3 laptop models. You should be purchasing business class hardware to reduce lineup changes. For example, Dell guarantees that each model in the Latitude line will be available for a certain amount of time, to give buyers a stable purchase option. This also gives you better support options geared towards IT departments.
You need to make sure that all IT related purchases get approved by IT, and you need to make sure you have a defined standard platform for employees. Get those in place, and I expect most of your problems will go away.
As to leasing . . . Be very, very, careful with leasing. It can be a very useful and important purchasing tool, but it can also get you into a huge amount of trouble.
First things first, never lease anything without a (reasonable) buyout option unless you are 100% absolutely guaranteed positive you are not going to keep it. Also, make absolutely sure that you keep track of your lease agreements and know what is and isn't on lease, and when the leases end.
Especially for smaller companies, I'm a fan of leasing for large purchases ($100k for a new storage system, or $200k for a new Oracle RAC setup, or whatever). It can help you spread the purchase out over a couple of years, instead of taking a big hit all at once.
I'm generally not a big fan of leasing on smaller purchases. Leasing adds additional overhead in order to manage the leases and track the systems. If you screw up here, you can end up with missing or lost systems that you then have to pay for (possibly at a premium) because you didn't own it, just leased it. And that's on top of the replacement cost you'll have to cover if you still need the device.
Part of the reason I feel so strongly about this is that I worked at a company that started with leasing a few things. Then some absolute morons in the finance department decided to lease everything. Without making that clear to the rest of the company.
By the time we figured out what was going on, we had devices that we'd paid for 2-3 times over (because we'd been leasing them for 5 years when they were purchased on a lease that covered their value in 18-24 months). Worse, the leases we'd gotten suckered into (again, complete morons in the finance department) had no buyout option. We had to rip out and return basically everything we'd purchased in the past few years unless we wanted to keep paying for it forever. And remember what I said about keeping track of everything? Hah! Right. It took roughly two full time employees six months to get a reasonable handle on where we were, with a huge effort to fully catalog and organize everything.
Oh, and also be very careful with leasing and software. You can really screw yourself over if you lease software (if purchased bundled with the hardware or via a third party leasing agent). You might have planned on upgrading your Exchange servers and keeping the same Exchange licenses, but if you bought them bundled with your Dell servers on a lease, and are returning your servers, those Exchange licenses are probably going to have to go back, too (also illustrating the importance of a buyout option again).
Leasing can be very useful. Just remember: Buyer beware, and make sure you know what you're doing. Remember that when you lease, you don't actually own the hardware, so keep track of it. Make sure you have a buyout.
Good luck. ;-)